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Re: dict comprehension

Terry Reedy

2/1/2008 6:21:00 AM


"Daniel Fetchinson" <fetchinson@googlemail.com> wrote in message
news:fbe2e2100801312151x7136d1bbu3498d7d21728100e@mail.gmail.com...
| Hi folks,
|
| There is a withdrawn PEP about a new syntax for dict comprehension:
| http://www.python.org/dev/peps... which says:

I believe both set and dict comprehensions will be in 3.0.



6 Answers

Arnaud Delobelle

2/1/2008 7:14:00 AM

0

On Feb 1, 6:21 am, "Terry Reedy" <tjre...@udel.edu> wrote:
> "Daniel Fetchinson" <fetchin...@googlemail.com> wrote in message
>
> news:fbe2e2100801312151x7136d1bbu3498d7d21728100e@mail.gmail.com...
> | Hi folks,
> |
> | There is a withdrawn PEP about a new syntax for dict comprehension:
> |http://www.python.org/dev/peps/pep-... says:
>
> I believe both set and dict comprehensions will be in 3.0.

Python 3.0a1+ (py3k:59330, Dec 4 2007, 18:44:39)
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> {x*x for x in range(10)}
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>> {x:x*x for x in range(10)}
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>>

That's nice.

--
Arnaud

Wildemar Wildenburger

2/1/2008 1:39:00 PM

0

Arnaud Delobelle wrote:
>> I believe both set and dict comprehensions will be in 3.0.
>
> Python 3.0a1+ (py3k:59330, Dec 4 2007, 18:44:39)
> [GCC 4.0.1 (Apple Inc. build 5465)] on darwin
> Type "help", "copyright", "credits" or "license" for more information.
>>>> {x*x for x in range(10)}
> {0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>>> {x:x*x for x in range(10)}
> {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>
OK, not bad. But I don't really see how this is better than the
generator approach.

Also, what is that first thing? A valueless dict (and thus a set)?

/W

Stefan Behnel

2/1/2008 5:30:00 PM

0

Wildemar Wildenburger wrote:
> Arnaud Delobelle wrote:
>>> I believe both set and dict comprehensions will be in 3.0.
>>
>> Python 3.0a1+ (py3k:59330, Dec 4 2007, 18:44:39)
>> [GCC 4.0.1 (Apple Inc. build 5465)] on darwin
>> Type "help", "copyright", "credits" or "license" for more information.
>>>>> {x*x for x in range(10)}
>> {0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>>>> {x:x*x for x in range(10)}
>> {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>
> OK, not bad. But I don't really see how this is better than the
> generator approach.

It's a literal syntax, just like you would do with a list, i.e. a list
comprehension. Why should you have list comps and no dict comps?


> Also, what is that first thing? A valueless dict (and thus a set)?

It's the literal set syntax added in 3.0. You can write

{1,2,3}

to get a set() or

{1:1,2:2}

to get a dict().

Stefan

Steven Bethard

2/2/2008 6:07:00 PM

0

Wildemar Wildenburger wrote:
> Arnaud Delobelle wrote:
>>> I believe both set and dict comprehensions will be in 3.0.
>>
>> Python 3.0a1+ (py3k:59330, Dec 4 2007, 18:44:39)
>> [GCC 4.0.1 (Apple Inc. build 5465)] on darwin
>> Type "help", "copyright", "credits" or "license" for more information.
>>>>> {x*x for x in range(10)}
>> {0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>>>> {x:x*x for x in range(10)}
>> {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>
> OK, not bad. But I don't really see how this is better than the
> generator approach.

It's more than twice as fast:

>>> setup = "items = range(10)"
>>> timeit.Timer("dict((x, x * x) for x in items)", setup).timeit()
6.0559464959932114
>>> timeit.Timer("{x:x * x for x in items}", setup).timeit()
2.8347301821879682

It also doesn't build the unnecessary intermediate tuples:

>>> def dict_genexp(items):
.... return dict((x, x * x) for x in items)
....
>>> def dict_comp(items):
.... return {x:x * x for x in items}
....
>>> dis.dis(dict_genexp.__code__.co_consts[1])
2 0 LOAD_FAST 0 (.0)
>> 3 FOR_ITER 21 (to 27)
6 STORE_FAST 1 (x)
9 LOAD_FAST 1 (x)
12 LOAD_FAST 1 (x)
15 LOAD_FAST 1 (x)
18 BINARY_MULTIPLY
19 BUILD_TUPLE 2
22 YIELD_VALUE
23 POP_TOP
24 JUMP_ABSOLUTE 3
>> 27 LOAD_CONST 0 (None)
30 RETURN_VALUE
>>> dis.dis(dict_comp.__code__.co_consts[1])
2 0 BUILD_MAP 0
3 DUP_TOP
4 STORE_FAST 1 (_[1])
7 LOAD_FAST 0 (.0)
>> 10 FOR_ITER 21 (to 34)
13 STORE_FAST 2 (x)
16 LOAD_FAST 1 (_[1])
19 LOAD_FAST 2 (x)
22 LOAD_FAST 2 (x)
25 BINARY_MULTIPLY
26 ROT_TWO
27 LOAD_FAST 2 (x)
30 STORE_SUBSCR
31 JUMP_ABSOLUTE 10
>> 34 RETURN_VALUE

STeVe

Bearophile

2/2/2008 9:11:00 PM

0

Steven Bethard:
> It also doesn't build the unnecessary intermediate tuples:

I see, but can't the interpreter improved to remove similar
intermediate tuples anyway? Is this a difficult thing to do?

Bye,
bearophile

Arnaud Delobelle

2/3/2008 9:03:00 AM

0

On Feb 2, 9:10 pm, bearophileH...@lycos.com wrote:
> Steven Bethard:
>
> > It also doesn't build the unnecessary intermediate tuples:
>
> I see, but can't the interpreter improved to remove similar
> intermediate tuples anyway?

Do you mean the compiler?

> Is this a difficult thing to do?

Yes, due to the HDNP (Highly Dynamic Nature of Python).

--
Arnaud